1. Field of the Invention
The present invention relates to a method and system for acquiring magnetic resonance (MR) data in magnetic resonance (MR).
2. Description of the Related Art
An atomic nucleus, having a spin and a magnetic moment which are not zero, in a static field, resonance-absorbs and radiates only an electromagnetic wave with a specific frequency by a magnetic resonance phenomenon. This atomic nucleus resonates at an angular frequency .omega.0 (.omega.0=2.pi..nu.0; .nu.0 is the Larmor frequency) defined as follows: EQU .omega.0 =.nu.H0
where .nu. is the gyromagnetic ratio which is inherent for each kind of an atomic nucleus, and H0 is the intensity of a static field.
In a magnetic resonance imaging (MRI) apparatus for diagnosing living bodies utilizing the magnetic resonance phenomenon, an MR signal induced after resonance absorption is detected and processed to acquire, without invasion, diagnosis data such as a slice image (MR image) of subject in accordance with, e.g., a density of atomic nuclei, longitudinal and transverse relaxation times periods, flow, and chemical shift.
Diagnosis data can be obtained on the basis of an MR signal generated by exciting the entire subject placed in a static field. In the conventional MRI apparatus, however, diagnosis data is obtained on the basis of an MR signal generated by exciting a specific portion of the subject in accordance with various limitations of the apparatus used and a specific clinical demand for an MR image.
In such an MRI apparatus, in order to reduce an artifact of an MR image due to motion and blood flow of a subject, beat-sync scanning is performed. In beat-sync scanning, for example, an R-wave of an electrocardiogram signal detected from the subject, on whom an electrocardiogram lead electrode is mounted, is used as a scanning sync signal, and MR data is acquired in accordance with the scanning sync signal.
Scanning for acquiring MR data in accordance with a beat sync signal to reconstruct one MR image will be described hereinafter with reference to FIG. 1.
As shown in FIG. 1, after a predetermined period of time (application timing period) Td elapses from generation of an R-wave of an electrocardiogram signal of a subject P, a trigger signal is generated to apply an RF (radio frequency) pulse by a spin echo method. The RF pulse is applied to a slice portion SL of the subject P in response to the trigger signal. An MR signal generated from the slice portion SL upon application of the RF pulse is acquired as MR data in a first encoding process. When RF pulses are sequentially applied to the slice portion, MR data of second, third, . . . encoding processes are acquired.
For example, when an MR image having a 256.times.256 matrix is reconstructed, application of an RF pulse is repeated 256 times at a timing of generation of each trigger signal. Therefore, MR data associated with 256 encoding processes are acquired.
Note that, when intervals between adjacent R-waves are constant, a repetition time Tr of the RF pulse is also constant. However, when the intervals between R-waves are changed due to, e.g., arrhythmia in the fourth beat, the repetition time Tr of the RF pulse is changed into a repetition time Tr'.
When the repetition time Tr of the RF pulse is changed, the amplitude and phase of the MR signal are changed. MR data is changed in an encoding direction on an MR image in accordance with a change in amplitude and phase of the MR signal. Therefore, an artifact occurs in the encoding direction.
An averaging process is not taken into consideration in the above-mentioned MR data acquisition. However, when MR data is acquired in practice, an averaging process is performed in order to improve the S/N ratio. More specifically, a plurality of MR data acquired in a single encoding process are averaged. Therefore, the amplitude of a normal MR signal is always constant.
On the other hand, since the amplitude of a noise signal is irregular, the amplitude of the added noise signal is much less than that of the normal MR signal. Therefore, when the noise signal is averaged together with normal signal components, only a substantially normal MR signal can be acquired, thereby improving the S/N ratio.
However, during acquisition of MR data, if cardiac arrhythmia occurs in a subject or the subject moves, the amplitudes of MR signals obtained by the single encoding process are changed. Therefore, when the averaging process is performed, an S/N ratio is reduced.
In addition, when a change in the subject, e.g., cardiac arrhythmia or motion, is not monitored, an operator cannot determine whether or not the acquired MR image includes an artifact caused by a change in the subject.
Furthermore, if cardiac arrhythmia occurs in the subject or the subject moves, MR data must be reacquired. Therefore, it takes a long time period to acquire MR data.
Thus, an MRI apparatus is desirable which can show a change result of a subject to an operator if cardiac arrhythmia occurs in the subject or the subject moves during an acquisition of MR data, and which can reacquire MR data in the same encoding process when the subject is changed.